The present invention relates to protection devices, and more particularly to an apparatus and method for representing protection device trip response.
The actual tripping of many protection devices, including some types of circuit breakers, overload relays and fuses, is generally dependent on the magnitude of the current, time, and the energy.
Referring now to FIG. 1, a current I versus a time t is shown for a particular protection device wherein a wave-form 20 is generally sinusoidal. The current must reach a particular threshold 22 for the device to begin responding, and then there must be enough energy to drive the tripping mechanism (e.g., magnets, bi-metals, melting conductors, blow-open contact arms, summing electronic trip unit elements, etc.) to complete the trip action. A quantitative representative value for the energy is expressed as I2t, which is an integral function shown in FIG. 1 as the area of a region 24 under wave-form 20 and above threshold 22.
The energies (and the corresponding quantitative representative values) become particularly important in certain high current transient conditions because the wave-forms can be non-sinusoidal thereby resulting in a larger or smaller energy region. For example, and referring now to FIG. 2, current I versus t time for the protection device represented in FIG. 1 is shown wherein a wave-form 26 is non-sinusoidal. The threshold current 22 is generally the same for the particular protection device. However, a region 28 can be of a smaller area than region 24, such that the energy represented by region 28 is insufficient to drive the tripping mechanism.
A plurality of protection devices in series is used to provide system selectivity. In general, a selective system is one in which the device or devices nearest to the fault trip with limited disruption of upstream protection devices. A series combination of protection devices is shown schematically in FIG. 3. FIG. 3 generally shows a two tier selective system 30. Selective system 30 comprises a source 32, an upstream protection device 34, and a downstream protection device 36 coupled to a load 38. Any number of additional downstream protection devices with corresponding loads may be included in system 30.
The let-through and trip time may be influenced by the series combination, as shown in FIG. 4. More particularly, trip curve 40 represents the behavior of a particular device used alone, whereas trip curve 42 represents the behavior of the same device employed as upstream protection device 34 with a corresponding downstream device 26 in series therewith.
It would, therefore, be desirable to provide a robust method and apparatus for analyzing protection devices.
It would also be desirable to provide a method and apparatus for demonstrating selectivity.
A method and apparatus generates an enhanced trip time curve capable of capturing both the non-sinusoidal energy and series effects.
In one embodiment, a method for representing trip times for a protection device is provided. The method includes plotting a time on a y-axis as a function of current on an x-axis and an energy representation on a z-axis. In a preferred embodiment of the method for representing trip times for a protection device, a quantitative representative value for the energy is expressed as I2t.
In another embodiment, a method for representing on times for a protection device is provided. The method includes plotting a time on a y-axis as a function of a peak let-through current value on an x-axis and a corresponding energy on a z-axis. In a preferred embodiment, a peak let-through current value is converted to a mapping current value by employing a function using the peak let-through current as the independent variable.
In a further embodiment, a method for determining selectivity in a multi-tier electrical distribution system is provided. The method includes plotting a trip time on a y-axis as a function of current on an x-axis and an energy representation on a z-axis, and further, plotting a clearing time on the y-axis as a function of a peak let-through current value on the x-axis and a corresponding energy on the z-axis. In a preferred embodiment, the a peak let-through current value is converted to a mapping current value by employing a function using the peak let-through current as the independent variable. An intersection between the trip response surface and the let-through surface represents selectivity limit.
In a further embodiment, the current and energy values are from software generated data, empirical data from laboratory experiments, empirical data from actual operations, theoretical data, hypothetical data, or any combination of the aforementioned data types.